Perchlorate salts have been and are currently used in solid rocket propellants, such as for launching space shuttles and military missiles. Other uses and sources of perchlorate also exist. Use of perchlorate has resulted in contamination of soil and groundwater. This contamination is of concern because of its associated health risks. The state of California, for example, has a maximum contaminant level (MCL) of 6 parts per billion in water. Perchlorate salts are very soluble in water and thus are capable of being transported long distances in groundwater aquifers. For example, several perchlorate plumes in California are miles in length. Perchlorate in soil can easily be leached into groundwater by infiltrating stormwater. These perchlorate sources in soil overlying groundwater are thus of great concern. Continuing contamination of groundwater by perchlorate sources in soil will result in the need to treat groundwater for extended time periods at greater costs. Treatment of perchlorate sources in soil will result in faster cleanup of groundwater and decreased costs.
In addition, other contaminants that have negative health impacts can leach from soil into groundwater. These contaminants include but are not limited to nitrate salts, nitrite salts, hexavalent chromium compounds, selenium compounds, and chlorinated hydrocarbons.
Several technologies are available for shallow soil treatment. Shallow soil can be excavated and disposed in a landfill. Anaerobic biological treatment is also possible for treatment of shallow soil in situ or ex situ. Technologies for treatment of deep soil are extremely limited and have not been successful. For example, addition of liquid electron donors to deep soil by percolation has been considered but has been limited by channeling of the liquid electron donors through the soil and incomplete contact between electron donor, contaminant (particularly perchlorate), and bacteria. Other technologies that are applicable to deep soil remediation are not generally applicable to the treatment of perchlorate. These technologies include in situ thermal treatment, aerobic biological treatment or bioventing, and soil vapor extraction. Thermal treatment is not applicable to perchlorate because it is stable at typical thermal treatment temperatures. Aerobic biological treatment of bioventing is not applicable to perchlorate because it is biodegraded only under anaerobic conditions. Soil vapor extraction is not applicable to perchlorate because it is not volatile.
The prior art also describes the use of gaseous electron donors, including hydrogen, ethyl acetate, and hexene to promote in situ anaerobic bioremediation of perchlorate and other contaminants in soil. Nevertheless, there is still a need for new methods of bioremediation employing a gaseous electron donor that provides improved treatment of perchlorate contamination and/or other contamination.